Current limiting apparatus



May 19, 1953 F." KESSELRING CURRENT LIMITING APPARATUS Original FiledAug. 1, 1946 5a 5a 5a 53 53 ?l ,l l 51 {I v .r'

I lxd/ I I I I I. ,LIIL -i I, t v i I; Is I; Id 1',

2 INVENTOR FRITZ KESSELRIN a Patented May 19, 1953 UNITED STATES PATENTOFFICE Original application August 1, 1946, Serial No.

687,625, now Patent No. 2,619,563, dated November 25, 1952.

Divided and this application December 5, 1951, Serial No. 259,993. InSwitzerland August '7, 1945 6 Claims.

This is a division from my copending patent application Serial No.637,625, filed August 1, 1946; for Electromagnetic Control Device, nowPatent No. 2,619,568.

My present invention relates to current limiting apparatus fordirect-current and alternoting-current power circuits.

It is an object of my invention to devise current limiting apparatuscapable of reducing or limiting excessive overloads, such asshort-circuit currents in power lines, at a velocity of current reducingoperation superior to that heretofore obtainable.

Another object is to provide current limiting apparatus affording aselective disconnection or sectionalization of overloaded portions inmeshed power-line networks.

A current limiting apparatus according to the invention has at least oneelectromagnetically actuated switch element or armature whose switchinggaps are paralleled by an electric impedance. Each switch element is anelastically suspended rigid body of at most 1 cm. length in thedirection of the magnetic field and has a switching travel betweenclosed and tion are preferably provided with a plurality of switchmembers or armatures arranged electrically in series but magnetically inparallel relation to each other, each armature having an impedance meansconnected parallel thereto. The current limiting apparatus, as a rule,do not have to completely interrupt the current but may merely reducethe current to a residual value readily controllable by a separatedisconnect switch. The impedances may be dimensioned to reduce theresidual current at least to the rated current value of the circuitinterrupting switch, or the impedances values may be large enough toreduce the current to such a. value that the are between the partingcontact surface of the switch extinguishes by itself. The impedances maybe purely ohmic resistors, free of self-induction, made of thin wires orof banda first maximum shaped conducting material with a positivetemperature resistance coeflicient, such as pure iron or tungsten. Theseresistors are preferably arranged in an insulating and cooling liquidwithin the housing of the current limiting apparatus. Under certainconditions it is also possible to use capacitive impedances whichautomatically, a few hundredths of a second after their insertion acrossthe contacts, reduce their capacity to a fraction of its original value,for instance, by means of increasing the distances between the capacitorelectrodes.

These and other objects and features of my invention will be apparentfrom the following description in conjunction with the drawing in whichFig. 1 shows schematically and in section a current limiting apparatusaccording to the invention, and Fig. 2 is an explanatory coordinatediagram of current and voltage curves.

The apparatus illustrated in Fig. 1 has 'a magnetic structure with afield coil 51. lhe structure 50 has two parallel pole faces betweenwhich a plurality of armatures 52 are pivotally mounted on torsionalsprings 49 which bias each armature against stationary contacts 53. Thehousing of the apparatus forms a tank 54 beneath the structure 5c and isequipped with insulating bushings 55, 56 for two respective lineterminals. A resistor 57 made of fiat iron strips is immersed ininsulating oil '58 contained in the tank 54. The line terminals areconnected by leads 59 and 6!] with the two outermost stationary contacts53. The resistor 51 has taps connected by leads 6! with the armatures 52and with the intermediate stationary contacts 53. Relative to the lineterminals, the armatures 52 are electrically series arranged. Relativeto the magnet field, the armatures lie parallel to one another forsimultaneous operations.

When the current limiter is traversed by load current, this currentflows from the terminal of bushing 56 through lead 6i! and seriallythrough all stationary contacts 53 and armatures 5.2 to lead 59 and theterminal of bushing 55. sufiiciently, energized to move the armaturesagainst their bias. However, when a heavy overload such as a shortcircuit occurs, the field of coil til causes all armatures to move tothe open positions. in the most common case, the excitation current offield coil 54 is provided by the short-circuit current itself, in whichcase the excitation current is tapped from across a gap in lead 59 asshown by dotted lines. It is Normally, the coil 5! is not, or not alsopossible to use auxiliary devices for controlling the switching of theresistors in ac-' cordance with the special character of theshortcircuit current. The ohmic value of this resistor is dimensioned toproduce an intense heating effect so that, due to the high positivetemperature coefiicient of resistance, the ohmic value increasesconsiderably. By employing chemically pure iron as material for theresistor, an increase of 5 to times its rated value becomes effective.By this means a further increase of the overload current is prevented.

This operation of the limiter will be more fully understood from thefollowing description of a typical performance represented by Fig. 2.

In Fig. 2, the normal, rated current of the limiter is indicated by awave 2' Curve z' denotes a surge of a short-circuit current as it mightappear when using conventional overload breakers. Curve i denotes thecurrent through a parallel connected impedance according to theinvention, and curve Uw represents the transient voltage drop acrossthis impedance and hence the restriking potential. A few 10 see. afterthe initial occurrence of overload current (instant to.) the armaturecontacts are opened so that the parallel connected impedances (resistor51 in Fig. 1) are inserted into the load circuit. The initial voltagedrop in the parallel connected impedance must not amount to more thanvolts, in order that an are between the contact shall either not occurat all or shall at once be extinguished. Due to heatin of the resistor51, the effective resistance increases. Hence, the voltage drop alsoincreases. The ccntact separation must increase to the same extent inorder to avoid a return ignition. Finally, almost the whole voltage ofthe circuit is impressed across the impedance, and

the current is reduced to a fraction of its normal value and can bereadily interrupted by a disconnect switch. By suitably dimensioning theimpedances, it is possible to limit the amplitude of the current z',,,,for example, to 1 /2 or twice the value of the rated current of thelimiter (instant in). The movement of the contacts, as explained, is sorapid that the dielectric strength between the contact surfaces issufficient to withstand the maximum of the restriking voltage (instantis). At about the instant is the disconnect switch may be opened tofully interrupt the current 2' at the instant te. Indeed, such a limitercan readily be made to begin opening its contacts not later than about10 sec. after the occurrence of .a surge front (instant ta) and to passthrough because it was found that arcs are extinguished on the line whenthe current is reduced to a certain minimum value and when the fullmagnitude of the above-mentioned short-circuit current is prevented fromflowing through this arc. The are on the line is then extinguished atabout 'the moment is (Fig.- 2) whereupon the current limiter at oncereturns to its closed position. In this case, the trouble is eliminatedin the shortest possible time without complete interruption of the linecurrent, and the consumer may not even be aware of a disturbance on hisline.

In case of a metallic short circuit all current limiters in the shortcircuit current path will be affected and therefore insert theirresistors. If the system of the power network is laid out to operate thelimiter at the location where the voltage decreases to zero, a selectiveisolation of the short circuit is established at the instant ha and atthe same time a short-circuit current of full magnitude is prevented.

Generators, transformers and the like devices subject to damage byoverloads may be equipped from the outset with a current limitingaccording to the invention. The general use of such devices equippedwith such current limiters, when the already existing line switches canbe used for the normally required control operations, permits aconsiderable simplification in the design of power line networks, sincethermal as well as dynamic damages due to short-circuit currents arevirtually eliminated.

It should be understood that the illustrated embodiment is presented forexemplification rather than limitation and may be varied and modifiedwithout departing from the essential features of my invention and withinthe scope of the claims annexed hereto.

I claim:

1. Current limiting apparatus for direct-current or alternating-currentpower circuits, comprising terminals for connection in a circuit whosecurrent is to be limited, a magnetic circuit having a magnet with a coiland having at least one armature disposed in the field of said magnetand movable between two given positions. spring means connected withsaid armature and having at said armature a force opposed to andnormally stronger than that of said field so as to normally hold saidarmature in one of said positions so that said magnet field is effectiveto move said armature to said other position only when said current hasan excess value, stationary contact means engaging said armature in saidone position and being connected through said armature and in serieswith said coil across said terminals, said armature having in thedirection of said field a maximum length of one centimeter and having amaximum travel between said positions of one millimeter and a maximumtravel period of 10- seconds, impedance means connected between saidstationary contact means across said armature, said magnetic circuithaving a time constant below 10* seconds so that the armature-movingvariation of excitation of said magnetic circuit takes place at thelatest a few 10- seconds after the initiation of the building-up of saidexcess current value, whereby contact separation occurs at most 10*seconds after said initiation and at least half said travel is traversedbefore the restriking potential corresponding to the voltage drop atsaid impedance means attains its first maximum value.

2. In apparatus according to claim 1, said impedance means beingsubstantially free of inductance and having a positive coefiicient ofresistance to increase the resistance value during circuit openingperformance.

3. In apparatus according to claim. 2, said impedance means beingcomposed of chemically substantially pure iron.

l. In apparatus according to claim 1, said impedance means having amaximum voltage drop of 20 volts at the moment of contact separation.

5. Current limiting apparatus, comprising two mutually insulated lineterminals, an electromagnet having two pole faces spaced from each other.8 to provide a magnetic field, a plurality of annatures disposedbetween said pole faces in magnetic parallel relation to each other andbeing individually biased to one position and magnetically movable dueto said field to another position, stationary contact means engageableby said armatures and connecting said armatures in series relation toeach other across said terminals in said one position of said armatures,each of said armatures having substantially in the direction of saidfield a maximum length in order of one centimeter and having a maximumtravel distance between said two positions in the order of onemillimeter and a maximum travel interval between said positions in theorder of 10- seconds, and impedance means connected across each gap ofinterruption between said armatures and said contact means, saidimpedance means having a maximum voltage drop of about 20 volts acrosseach of said gaps during said interval.

6. Apparatus according to claim 1, comprising a tank with coolingliquid, said impedance means consisting of ohmic resistors and having aposi- 1c tive temperature coeiiicient of resistance, and

said resistors being immersed in said liquid.

FRITZ KESSELRING.

N 0 references cited.

